1. Field of the Invention
The present invention relates to a differential protective relay apparatus in which a plural-branch bus bar including a plurality of terminals is differentially protected via a current transformer provided in the lines respectively.
2. Description of the Prior Art
FIG. 1 shows a system connection diagram of a differential protective relay apparatus. Symbol "0" indicates a bus bar; numerals 1 to n denote terminals drawn from the bus bar "0", numerals CT 11 to CT 1n represents current transformers (simply referred to as a "CT" hereinafter) connected to the respective terminals 1 to n; and symbol "Z.sub.D " represents an impedance of a differential circuit 20 which is shunted by parallel-connection of secondary circuits of the above-described CT 11 to CT 1n.
In general, a differential protective relay apparatus may be categorized into two typical differential types, i.e., a high impedance differential type and a low impedance differential type by selecting either a high impedance, or a low impedance as the impedance "Z.sub.D " of the above-described differential circuit 20.
In accordance with the former high-impedance differential type, since the differential circuit is shunted by the relatively high impedance Z.sub.D, the shunted current components from the current transformers CT 11 to CT 1n which are differentially connected are small and only a small amount of energy is transferred. As a result, in the case that the currents flow into the differential circuit 20 in the same direction upon an internal fault, a relatively high voltage appears across the impedance Z.sub.D of the differential circuit 20. Whereas, in the case that the shunted current is circulated through CT 11 to CT 1n which are differentially connected upon an external fault, the lead wire resistances of the secondary circuits of the differentially connected CT 11 to CT 1n are added to the exciting impedance of the CT at the current flowing terminal on the external fault, and thus the voltage drops appearing across the resistances do not exceed a value determined by the voltage drop vs current of the CT exciting characteristics.
In, on the other hand, the low impedance differential apparatus, a large amount of the shunted current component flows from the differentially connected CT 11 to CT 1n to the impedance Z.sub.D, and thus most energy is transferred to the differential circuit side.
As a consequence, the voltage appearing across the impedance of the differential circuit does not go up so high in case of an internal failure.
Whereas, on an external fault, the secondary exciting impedance of the CT 1n of the current flowing out terminal and the impedance of the differential circuit (Zd) show values of equal order or else the latter shows a lower value, which causes a larger shunt current through the differential circuit impedance Zd, by the voltage drop induced along the lead wire resistance of the CT secondary circuit of differentially connected CTs 11 .about. 1n-1.
As a consequence, there is a trend in the low impedance differential type scheme that malfunctions are apt to occur when a heavy external fault current flows.
In general, assuming that the resistance value of the differential circuit 20 is "R.sub.D ", and total resistance value in the reciprocating path of the lead wire and the like of the CT secondary circuit (the secondary winding resistance value "R.sub.S " of CT and also the resistance value "R.sub.L " of CT secondary lead wire) is "R.sub.2 ", when the maximum external fault current I.sub.FE max passes, both the differential circuit current "I.sub.D " and differential circuit voltage (i.e., a voltage across the impedance Z.sub.D) V.sub.D are expressed as follows. ##EQU1## The differential circuit voltage does not exceed over a predetermined value.
On the other hand, in case of an internal fault, when the voltage "V.sub.D " of the differential circuit appearing across the impedance Z.sub.D is detected by the setting sensitivity of the applied voltage V.sub.S, a minimum internal fault detecting current is given by: ##EQU2## It should be noted that "I.sub.ex (V.sub.S)" is a secondary exciting current with respect to the applied voltage, and "n" denotes the number of connected lines for the protected bus bars.
Since the conventional high impedance differential protective relay schemes are arranged in the above-described configurations and operations, the high impedance differential apparatus has the following problems.
Although the differential relay can be set to prevent false operation on external faults below the value expressed as in the formula (1.3), the minimum pick-up current of an internal fault should satisfy the formula (1.4) as long as the relay is set to the value determined by the formula (1.3).
In other words, in the conventional apparatus, there are drawbacks that, when the number of terminals connected to the bus bar becomes large and furthermore the internal fault current is smaller, the minimum pick-up sensitivity is adversely affected by both the secondary excitation characteristic "I.sub.ex -V.sub.ex " of the current transformer and the number of terminals "n".